The use of high-power fiber-coupled lasers continues to gain popularity for a variety of applications, such as materials processing, cutting, welding, and/or additive manufacturing. These lasers include, for example, fiber lasers, disk lasers, diode lasers, diode-pumped solid state lasers, and lamp-pumped solid state lasers. In these systems, optical power is delivered from the laser to a work piece via an optical fiber.
Various fiber-coupled laser materials processing tasks require different beam characteristics (e.g., spatial profiles and/or divergence profiles). For example, cutting thick metal and welding generally require a larger spot size than cutting thin metal. Ideally, the laser beam properties would be adjustable to enable optimized processing for these different tasks. Conventionally, users have two choices: (1) Employ a laser system with fixed beam characteristics that can be used for different tasks but is not optimal for most of them (i.e., a compromise between performance and flexibility); or (2) Purchase a laser system or accessories that offer variable beam characteristics but that add significant cost, size, weight, complexity, and perhaps performance degradation (e.g., optical loss) or reliability degradation (e.g., reduced robustness or up-time). Currently available laser systems capable of varying beam characteristics require the use of free-space optics or other complex and expensive add-on mechanisms (e.g., zoom lenses, mirrors, translatable or motorized lenses, combiners, etc.) in order to vary beam characteristics. No solution exists that provides the desired adjustability in beam characteristics that minimizes or eliminates reliance on the use of free-space optics or other extra components that add significant penalties in terms of cost, complexity, performance, and/or reliability. What is needed is an in-fiber apparatus for providing varying beam characteristics that does not require or minimizes the use of free-space optics and that can avoid significant cost, complexity, performance tradeoffs, and/or reliability degradation.
The powder bed utilized for 3D metal printing (including DMLS, SLM, and SLS) provides very effective heat sinking of the laser processed region. This effectiveness results in extremely high cooling rates which serve to lock stresses into the material. Undesirable stresses can cause part deformation and may lead to defects that result in mechanical failure. To resolve this difficulty, 3D metal printed parts are frequently subjected to a post-printing bake out process to anneal or /stress relieve the part. The bake-out process raises the temperature of the part to a critical level and provides a time controlled cooldown sequence to relieve the buildup of undesired stresses, and is performed as a bulk operation, meaning that the entire part is annealed simultaneously.
In general, it is often desired to fabricate parts having areas of high hardness (such as for wear surfaces), and areas of softer material (such as for mechanical integrity). The hardening of certain materials may be achieved through controlled, rapid cooldown. Hardening intentionally locks in stress that effectively elevate the yield stress of the material. Such hardening process are also generally known to be carried out at a bulk level.
Selective methods for modifying the stress state of materials do exist. Examples include case hardening and differential tempering or hardening. Laser hardening of machined products is also known. However, the ability to selectively anneal and/or harden different regions within a component is limited with existing manufacturing technologies. Furthermore, annealing and hardening are typically performed as secondary or tertiary operations during the manufacturing process, thus requiring additional manufacturing steps that can be time consuming and/or costly.
Achieving desired stress states can be challenging for additive manufacturing processes because different materials used for additive manufacturing often have different rates of cool down based on their physical properties, such as thermal conductivity, laser absorption, porosity of final melt, and initial powder size and composition. Further, laser heating is often employed in additive manufacturing, in which constant laser beam size can further limit the ability of the part fabricator to counteract the challenges posed by differing materials and high cooling rates with desired control.
Therefore, additive manufacturing processes that allow improved tailoring of stress states in a three-dimensional object would be a welcome addition to the art.